The present invention relates to the field of optical tracking, and in particular to the field of laser target acquisition and tracking. More particularly, in various embodiments the present invention relates to a beam pointing and tracking apparatus and method for laser target acquisition and tracking using translational stages that move an optical fiber in the focal plane of the transmitting and receiving optical system while surrounded by a reflecting mirror in the focal plane that also moves in conjunction with the optical fiber.
Laser targeting and tracking systems are today employed in a vast array of military and civil applications. A laser targeting and tracking system generally consists of a transmitting terminal and a receiving terminal. A transmitting terminal transmits an optical signal generated by a source that converts electrical signals to optical signals for transmission out of the transmitting telescope. The receiving terminal receives the laser illuminated target “signature” signal (back reflection) into a receiving telescope, which focuses the optical signal into an optical photodetector, and then converts the light energy into an electrical signal.
Pointing the beam that exits an optical transmitter is typically carried out via a motorized beam-steering gimbal or mirror system that guides the laser beam through the telescope to the target. As with the transmitting telescope, the receiving telescope also uses scanning mirrors supported by gimbals to acquire and track the incoming optical signature. Gimbals are used to steer the mirrors in this sort of system. A gimbal is a mechanical apparatus to allow a suspended object to rotate freely along two orthogonal axes simultaneously, within a defined angle of view. Gimbals are well known in the art, having been used, for example, since at least as early as the sixteenth century in the suspension of maritime compasses. Accurate alignment of the laser targeting system is essential for free space laser target tracking systems. Thus such systems must provide accurate alignment and high angular resolution in order for the receiver telescope to efficiently collect the incoming optical beam. Conversely, the transmitter telescope must be able to accurately point its beam so that a remotely-reflecting object can efficiently reflect the optical signal for the receiver photodetector.
In addition to the gimbal-based systems described above, beam steering in optical systems may also be accomplished by other means. In particular, some existing non-gimbaled beam-steering solutions include acousto-optics, liquid crystals, electro-optics, micro-optics, galvanometer or magnetic mirrors, and micro-mirror arrays. These types of systems, however, have generally proven to be unwieldy, or lack the speed, precision, and reliability necessary for high-speed, long-distance laser target tracking. Thus the most common means for beam steering in optical communications systems remains by the use of a motorized gimballing system. A gimballing system used for the alignment of an optical transmitter or receiver typically moves the entire transmitting or receiving telescope through the required field of view.
Accurate alignment of the transceiver system is essential for laser target tracking systems. Therefore, gimballing systems must provide accurate alignment angular resolution in order for the receiver telescope to efficiently collect the incoming optical beam. Conversely, the transmitter telescope must be able to accurately point its beam so that a remote-receiving terminal can efficiently collect the optical signal for the photodetector. Mechanical gimballing systems have been favored in many laser tracking systems because they can provide very fast alignment times coupled with high angular resolution.
Gimballed beam-steering systems do, however, suffer from several important disadvantages. Such systems are quite heavy due to the weight of the mechanical components, motors, and servos necessary for such a system. While weight may not be as important a factor in the design of a land-based system, weight is of paramount importance in aircraft design, which is a critically important application for laser tracking systems. Gimballing systems are also quite bulky due to the required mechanical components, which is also a significant disadvantage in the design of airborne systems. Finally, mechanical gimballing systems require the use of a great deal of electrical power, far more power than is typically consumed by the electronics associated with an optical receiver or transmitter system. Again, while power consumption may not be as important a factor in permanent ground-based systems, it is a critically important factor in airborne systems, as well as in mobile ground-based systems such as may be mounted on land vehicles.
In order for an optical receiver to begin receiving a signal from a transmitter, the incoming search signal must first be located and the receiver pointed in the direction of the incoming signal. During the initial search for a signal, or if the signal is lost for some reason and reacquisition is thus necessary, a search pattern is generated by an algorithm stored in the receiver control system. The search pattern crosses the focal plane of the receiver or a specified sub-area, called the Uncertainty Area (UA), employing the motion control stage. This pattern may be raster, spiral, spoked, lissajous, rose, etc. as are known in the art. Each pattern has its own advantages and disadvantages. For example, the rectangular spiral pattern suffers a sacrifice in search loop-time as opposed to some contoured patterns, but has the advantage of a gain in redundancy by using a sequential motion system. That is, both the search pattern and tracking involve only one axis in motion at a given time, allowing (by use of a simple MUX circuit) each servo amplifier in a typical control system to be a backup for the others. Studies indicate that rose and lissajous search patterns are the most efficient in finding a satellite communication beam. Search time using these search patterns may be reduced by as much as half that required for a basic raster pattern. This is due to the fact that motor reversals (and associated velocity profiles, striction, etc.) are minimized, allowing more “ground” to be covered in a given time period. Using another example, an outward spiral search pattern is desirable if an acquired signal is suddenly lost, because it weights the search in the last known area of the beam.
In order for a search to be successful using the currently employed gimbal-based laser communications systems, the target terminal must be aligned with the scanning terminal to within its (instantaneous) field of view (FOV) to “see” the scanning terminal when it is swept by the scanning terminal's beacon. This means that a scan-stairstep-scan approach must be used whereby the scanning terminal sweeps its entire UA and, if detection is not made, the terminal then moves a small amount and waits for the scanning terminal to sweep its UA again. This process must be repeated until the terminal happens to have the scanning terminal within its FOV. This is a very inefficient process, sometimes requiring hours for acquisition. For example, looking at a purely statistical acquisition process for a typical GEO-GEO UA (say +/−2 degree UA) will result in a motion plane search of approximately 2 mm×2 mm. If one assumes a 60,000 km separation between terminals—as is common for satellite communications—and calculates the distance traveled in the motion plane per “flash-time” (i.e., time that the transmitted beam is incident on the receiver optics during its sweep) and call this a “step,” and then calculate an increment to move over per pass of the search pattern and still detect the target's beam, one can estimate how many loops (i.e., iterations of the search pattern) are needed for acquisition. With a typical laser communications system, a hypothetical 8 μm step and 10 μm increment is not unusual. This example would then generate approximately 50,000 discrete “points” that are the size of the effective FOV (instantaneous FOV plus the area that the FOV could be moved through during flash time) where the receiver could be positioned when the incoming beam sweeps its optics. If the search pattern contains no overlap, the odds are 50000:1 that an acquisition will occur during a given loop-time if no other information is known. At 1 ms per point, the loop-time taken would be 50 s and the required average traverse speed (feed rate) in the motion plane is 80 mm/s. This situation obviously would lead to long acquisition times and the need for complex acquisition processes to statistically “rule out” sections of the UA. It may be noted that feed rate is limited by the minimum flash time needed at the target to acquire the needed data. Data may be taken at higher speed by using faster detectors and high-speed analog-to-digital (A-to-D) converters. However, this can have detrimental affects to tracking performance.
Another problem is presented by attempting acquisition between terminals that are very far apart and/or are moving very fast relative to each other The additional challenges in this situation are due to the fact that the fast-moving beam requires high bandwidth signal sampling, and that transmitter jitter cannot be compensated before the link is established without the use of inertial sensors. Because of these issues, even the scenario described in the examples above may not work deterministically. The fact that the scanning motion must be very fast means that once a terminal detects its target's beam (sees a “flash”), it does not necessarily mean that its targeted terminal detected it simultaneously (due to differences in noise background, detector/processor latencies, etc), or that either could stop in time to lock-on before going off the other's FOV. Hence the best the scanning terminals can do is reduce their UA based on location of the flash. Also, because jitter cannot be compensated for before link acquisition, it is possible that the transmitter can miss the receiver optics at the time that acquisition should have occurred (without the jitter). Thus the entire process would have to be repeated. For applications in which Line of Sight (LOS) is always maintained, this issue may not present a serious problem. For missions in which LOS comes and goes, such as is common with orbiting satellites, this issue presents significant challenges.
It may be seen that if the UA can be reduced to an amount such that each terminal's beacon remains on the other terminal's optics throughout that terminal's search pattern, the terminals should acquire each other within one “loop-time.” Thus much effort has been expended to find ways to reduce the UA to this size. One technique for this is to widen (“spoil”) the beam as much as possible by using specially designed optics and/or reducing the focal length (i.e., using the Z axis of the motion control stage of typical fiber-optic based satellite communications systems to move the fiber closer to the telescope) during the search, and then reposition to the focal plane after acquisition. Widening the beam will make it more likely to fall on the optics of the other terminal, although this reduces the optical intensity. For acquisition, however, the intensity of the signal above the detection threshold is not consequential, since the receiver simply needs to be able to detect the signal, not read data error-free. Thus, it is most efficient to widen the beam to the limit that provides detectable intensity (which is a function of link distance and laser power) at the target terminal. For long distance links, this fall-off in intensity generally requires such a powerful laser that a separate system (laser and optics) is used for the search beacon from that of the communications system. This requires very large, expensive, and power-hungry systems.
What is desired then is a laser tracking system that provides high speed and high angular resolution, with reduced size, weight, and power consumption, and provides for rapid acquisition as compared to traditional gimballing systems now employed in such devices. These desires and met and the problems of the prior art are solved in the present invention, as described following.